as stated above; also. Part of a thermoneculear bomb. IT is sent into fusion by way of a fissile reaction. (Uranium bomb, plutonium gun) Then it procedes to be setn off by the extreme heat and presure into fusion to form He-3. The end result is KA FUCKING BOOOM!

Fat boy was a plutonium gun of about 750Kton. Mike was a tritium fusion bomb, about 10Mtonnes.